Human APOBEC3G is capable of altering the HIV genome by deaminating cDNA cytosines to uracils. This activity can genetically inactivate HIV. As a counter-defense mechanism, HIV promotes protein degradation of APOBEC3G. Degradation requires that the HIV virion infectivity factor (Vif) protein interacts with APOBEC3G. Current studies have revealed substantial genetic and biochemical details of this host-pathogen conflict, but an atomic level understanding is still lacking. Therefore, the major objectives of this research are to obtain a structural understanding of the DNA cytosine deaminase activity of APOBEC3G and of the APOBEC3G -Vif interaction. These objectives will be achieved through the following specific aims: (I) we will achieve an atomic-level understanding of APOBEC3G and how it catalyzes the DNA cytosine deamination reaction, and (II) we will obtain an atomic-level knowledge that will explain how APOBEC3G is recognized by Vif. These primary objectives will be met by combining genetic, biochemical, biophysical and structural (NMR) approaches. These studies will produce the first structure of a polynucleotide cytosine deaminase, and the resulting structural insights will take us several steps closer to achieve our long-term goal of developing therapeutic methods for enhancing APOBEC3G function. Human APOBEC3G (A3G) is capable of altering the HIV genome by deaminating cDNA cytosines to uracils. This activity can genetically inactivate the virus. As a counter-defense mechanism, HIV promotes the ubiquitin-mediated protein degradation of A3G. Degradation requires that A3G interacts with the HIV virion infectivity factor (Vif) protein. Current studies have revealed substantial genetic and biochemical details of this host-pathogen conflict, but an atomic level understanding is still lacking. Therefore, the major objectives of this research are to obtain a structural understanding of the DNA cytosine deaminase activity of A3G and of the A3G-Vif interaction.